Heat Index Calculator

Calculated using standard mathematical and industry formulas.

Free online Heat Index Calculator calculator and unit conversion tool for everyday, engineering, and home projects.

How to use this calculator?

1. Enter air temp ≥ 80°F.
2. Enter relative humidity %.
3. Read feels-like heat index and warning category.

Formula & calculation method

NWS Rothfusz Regression

HI = Multi-variable polynomial regression of Temp & RH

NWS heat index model.

Step-by-step calculation example

90°F at 65% humidity = 102.9°F Heat Index.

Key insights to remember

• High humidity prevents sweat evaporation.

How to interpret your results?

Heat Danger

NWS heat stroke alert category.

Frequently asked questions

How do I use the Heat Index Calculator calculator?

Enter your numerical input and select desired units to get instant accurate results.

How accurate are the conversions?

Conversions use official ISO conversion factors for maximum precision.

Does it convert metric and imperial units?

Yes, seamless conversions between metric (meters, kg) and imperial (feet, lbs) are supported.

Is my data stored?

No, all calculations execute 100% locally in your browser with zero data logging.

Can I use this for DIY or engineering projects?

Yes, it provides reliable estimates for construction, crafting, science, and everyday math.

What if I enter invalid inputs?

The calculator will display a friendly error prompt if non-numeric values are entered.

Related calculators

• Wind Chill Calculator — Calculate wind chill feels-like temperature and frostbite risk.
• Dew Point Calculator — Calculate dew point temperature and relative humidity.
• Temperature Converter — Convert between Celsius, Fahrenheit, Kelvin, and Rankine temperature scales instantly.

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